Malignant transformation by the rat-derived Harvey and Kirsten sarcoma viruses was mediated by a p21 protein coded for by the virus genome. This p21 protein possesses biochemical activities of GTP-specific autophosphorylation and guanine nucleotide binding. Efforts were undertaken to characterize in more detail these activities, and their possible role in regulating the pKm protein kinase which phosphorylates the Na+/K+ATPase. Biosynthesis of p21 was studied by taking advantage of the elevated synthesis in the SV40-HaSV recombinants.